An electroosmotic pump is a pump using a movement of a fluid by electro-osmosis which occurs when a voltage is applied to electrodes provided at two opposite ends of a capillary tube or a porous membrane.
Conventionally, platinum, which has high stability, has been widely used as a material of the electrodes. Recently, in order to drive the electroosmotic pump stably without accompanying gas generation, silver (Ag), silver oxide (AgO), MnO(OH), polyaniline (PANI), and the like are used as materials of the electrodes.
Among the materials used as the electrodes, a conductive polymer such as polyaniline is generally synthesized through polymerization whereby a monomer is oxidized electrochemically or chemically. Since the amount of positive charges within a polymer chain increases continuously during the polymerization, it is required to add negatively charged ions to the polymer chain so that the polymerization may progress continuously.
As the most general method of synthesizing a conductive polymer, a monomer such as aniline, pyrrole or thiophene is electrochemically oxidized or chemically oxidized by using an oxidizing agent in a solution of nitric acid (HNO3) or hydrochloric acid (HCl). The synthesis of the conductive polymer progresses as anions (negative ions) (e.g., NO3−, Cl−, or the like) that exist in the solution are continuously added to a polymer chain. That is, since the produced polymer chain itself is positively charged, the conductive polymer is synthesized as the anions are added to the polymer chain to establish a charge balance.
In this regard, Japanese Patent Laid-open Publication No. 2001-240730 (entitled “CONDUCTIVE POLYTHIOPHENE”) discloses polythiophene which is doped with molecular anions such as bis(perfluoro alkanesulfonyl)imide.
If a voltage is applied to each of a (+) electrode and a (−) electrode containing the conductive polymer produced as stated above, an oxidation-reduction reaction takes place in the entire polymer matrix, breaking a charge balance. Accordingly, to balance charges again, an ion having high mobility moves into the conductive polymer. By way of example, if polyaniline (PANI) is synthesized in a solution of nitric acid (HNO3), a polymer containing a molecular anion of NO3− is produced. If the polyaniline (PANI) in the form of an emeraldine salt (refer to Reaction Formula 1 below) containing this small molecular anion, positive (+) charges in the polymer chain disappear. Accordingly, to establish a charge balance, the anion NO3− once added to the polymer chain (A− in the following Reaction Formula 1) comes out of the polymer chain. As a result, the polyaniline (PANI) is converted into the form of a leucoemeraldine base and becomes a neutral polymer chain.

As stated above, in the oxidation-reduction reaction, the anion that has come out of the polymer chain for a charge balance of the conductive polymer contained in any one of the electrodes is moved to establish a charge balance of the conductive polymer of the other electrode. If a moving velocity of such an ion is slow, the oxidation-reduction reaction of the electrodes may not occur smoothly.
In case that an electrode reaction takes place in a solution which hardly contains electrolyte, it is difficult that the small anion (NO3−, Cl−, or the like) once moved into the conductive polymer of one electrode is mixed into the conductive polymer of the other electrode after coming out of the conductive polymer of the one electrode. Since the electroosmotic pump is mostly operated in a solution having a low concentration of electrolyte, if a material containing an anion having high mobility is used as an electrode, it is difficult to expect an electrode reaction to progress continuously. As a result, performance of the electroosmotic pump would be sharply deteriorated.
Further, generally, silica, glass, or the like is used as a material of a porous membrane included in the electroosmotic pump. A surface of the porous membrane made of these materials is negatively charged in an aqueous solution. Here, the anion that has come out of the conductive polymer during the aforementioned oxidation-reduction reaction of the electrodes should pass through the porous membrane to be moved between the electrodes. Since however, the surface of the porous membrane is negatively charged, a repulsive force is applied between the anion and the porous membrane, so that it is difficult for the anion to flow through the porous membrane. Therefore, the oxidation-reduction reaction of the electrodes may not take place smoothly, resulting in deterioration of the performance of the electroosmotic pump.
Meanwhile, as one of basic structures of an electrode, there is known a structure in which an electrode material as mentioned above (i.e., silver (Ag), silver oxide (AgO), MnO(OH), polyaniline, etc.) is electrodeposited on a carbon paper. Conventionally, by using this structure, a pumping force is generated by moving a fluid through an oxidation-reduction reaction of the material electrodeposited on the electrode.
In this regard, International Patent Laid-open Publication No. WO2011-112723 (entitled “ELECTRO-OSMOTIC PUMPS, SYSTEMS, METHODS, AND COMPOSITIONS”) describes an electroosmotic pump including electrodes on which silver (Ag) and silver oxide (AgO) are electrodeposited, respectively.
In case of the conventional electrodes, however, since a process of electrodepositing the other materials on the carbon paper is required, the whole process of preparing the electroosmotic pump has been complicated.